Green chemistry goes for gold

Tiny particles of gold have helped to clean up an important chemical reaction that is used every day to produce tonnes of pharmaceuticals, detergents and food additives.

Oxidation is the process by which oxygen atoms are added to carbon-based compounds to create these useful organic products. In principle, this can be done cleanly, using pristine oxygen gas (O2) from the air. But the chemical bond that bind its two oxygen atoms together must be broken so that they are free to react, and this is difficult, explains Graham Hutchings, a chemist from Cardiff University, UK. Instead, most oxidation reactions rely on harsh oxidants such as peracetic acid, which can produce toxic waste products.

Now Hutchings's team has discovered a way to make the clean process easier: the researchers found that gold nanoparticles, each just 25 nanometres across, can activate oxygen from the air at relatively low temperatures of 60 to 80 C and a gas pressure similar to that found in vehicle tyres.

"The chemical industry loves to take the temperature down because energy is such a large cost," says Richard Holliday, head of the industrial sector of the World Gold Council, which is headquartered in London and promotes the use of gold.

Joining in

The activated oxygen is easily added to carbon molecules. In many chemical reactions, liquids are used to help dissolve reactants and encourage atoms to combine, but the gold-activated oxygen can sometimes take hold without such solvents.

Doing away with solvents is one of the best ways to make industrial chemistry more environmentally friendly, Hutchings points out, since it reduces the amount of potentially-polluting liquid that has to somehow be disposed of. "If you can do something without solvent, it's inherently greener," he says.

The team collaborated with the London-based chemical company Johnson Matthey on the project. "They're very interested in scaling up these gold catalysts," says Hutchings, whose team reports its work in this week's Nature1.

Good things in small packages

Gold is normally inert, which is why it stays shiny and makes such good jewellery. "But if you take gold and divide it into clusters of a few hundred atoms it becomes incredibly reactive," Hutchings says.

In the catalyst, gold particles are stuck to larger motes of carbon, and Hutchings thinks that the interface between the two is the site where the oxidation reaction takes place. The team used the metal bismuth to tune the activity of the gold catalyst by blocking certain sites on its surface.

The researchers found that the catalyst was particularly good at making epoxides, where an oxygen atom forms a bridge between two carbon atoms.

"Epoxides are valuable chemicals, but they're not easy to make," says Hutchings. The cleanest method currently in use relies on hydrogen peroxide, commonly used as an antiseptic. Although this is relatively clean, Hutchings estimates that as an oxygen source it is eight times as costly as using air.

Other precious metals, such as palladium, are now widely used in chemical processes, but gold seems to be much more versatile, says Masatake Haruta, a gold chemist at the Tokyo Metropolitan University in Japan.